Aftermarket on-board diagnostics devices may be used to request diagnostic data from a vehicle that is not displayed to a driver. Typically, these devices are coupled to an electronic control unit (ECU) of the vehicle via an ECU/diagnostic connector of the vehicle. Data may be provided by diagnostic devices in the form of an on-board diagnostics parameter ID (PID), which is a code indicating diagnostic information. The PID may be interpreted or decoded according to a diagnostic standard, such as SAE J1979. However, PIDs traditionally cannot be directly retrieved from the ECU/diagnostic connector to provide a total energy capacity of the vehicle.